1. Field of the Invention
The present invention relates to a method for fabricating millimeter wave devices and, in particular, to such a method utilizing ion beam micromachining techniques.
2. DESCRIPTION OF THE PRIOR ART
In the prior art, insulating material from semiconductor devices is generally removed by means of chemical etching processes, for example, for purposes of producing lead through vias through the insulating material to the underlying device layer. Such processes produce irregular edges in the vias which, when leads are deposited therein, result in poor electrical performance and variation in performance from device to device, especially as the device size is reduced to 2 micrometers and smaller. One reason for this problem is that, as the frequency of electronic devices extends to the millimeter-wave range (1 GHz to 100 GHz and above), the performance characteristics of the devices are strongly influenced by the capacitive and resistive effects which are related to the device geometry, i.e., the device size, pattern sharpness, and film thickness of layers involved. For example, a Schottky barrier diode operates effectively at millimeter-wave frequencies. The diode configuration was first described by Kahng and Lepselter in Bell System Technical Journal, 44, 1525 et seq. (1965) and an improved design was described by Young and Irvin in the Proceedings IEEE 53, 2130-2131 (1965) in an article entitled "Millimeter Frequency Conversion Using N-type GaAs Schottky-Barrier Epitaxial Diodes with a Novel Contacing Technique". This configuration consists of small, well-defined metal contacts on an epitaxial semiconductor surface and a contact at the back or bottom of the device. A structure of insulating material, such as silicon dioxide, is provided over the semiconductor to define the device geometry, to stabilize the semiconductor surface, and to ensure that a sharpened wire securely connects one, and only one, contact pad on the surface.
When either the pattern edges for such metallic contact pad or the thickness of the epitaxial layer is imprecise, parasitic impedances arise and degrade the performance and operating frequencies of the diode. These impedances are identifiable as the shunt susceptance due to junction capacitance and the series and junction resistance due to the undepleted epitaxial material. The effect of the junction capacitance is to cause the high-frequency signal to bypass the junction resistance, which results in loss in rectified signal. Similarly, the series resistance degrades the output and includes the combination of the resistance of the undepleted material in the epitaxial layer, the spreading resistance of the substrate material due to the comparative physical sizes of the diode and the back contact, and the back contact resistance.
The depletion layer or junction capacitance arises from the space-charge associated with the potential barrier of the junction. It can be minimized by decreasing the size of the contact area and using a lightly doped epitaxial layer. Since it is dependent on the effective barrier potential, it is also a function of the electrical biasing.
The junction resistance of the diode, often called the barrier resistance, arises from the existence of a barrier potential which restricts the passage of current carriers.
The resistance due to the undepleted region of the epitaxial layer is considered to be the largest series resistance, primarily because of the small cross-sectional area involved and the higher resistivity of the epitaxial layer. For optimum operation, no undepleted material is desired; however, this situation is, in practice, difficult to attain. When the diode is normally forward biased, some of the depletion layer is exposed, that is, a portion of the formerly non-conductive layer becomes conductive, the remaining portion significantly contributing to the series resistance. If the epitaxial layer is sufficiently thin to be the same thickness as or less than that of the depletion region at the quiescent biasing point, the series resistance resulting from the undepleted region would be minimized. Therefore, it is desirable to have a very thin epitaxial layer with the highest conductivity possible, while taking into consideration the depletion layer capacitance.
The spreading resistance of the substrate material is determined by the resistivity of the substrate and the diameter of the junction. The resistivity of the substrate can be specified to be a very low value, and techniques such as thinning by mechanical lapping can decrease the path length of material contributing to the resistance.
In the above example relating to the Schottky barrier diode, the fabrication of such a device included two operations of thinning the epitaxial layer and the production of holes through an insulating layer for purposes of making connection to a contact pad. As stated above, both thinning and production of holes has been conventionally performed by chemical etching processes. The success of etching metallic or other films is dependent upon the elimination of undercutting in order to avoid parasitic leakage and to maintain sharply defined lines below the masking patterns. Chemical etchings produce serious undercutting problems and/or ragged edges because control of the etch rate is poor. Undercutting results in several problems, such as variable resistance, exposure of junctions and of device components otherwise intended to be protected, non-uniform capacitance, and discontinuity of deposition of material over the device. Ragged edges result in uneven capacitance and resistance and discontinuity of material deposition.
In prior art processes, plasma sputtering techniques are utilized; however, because the wafer is placed in the plasma, the plasma etching characteristics cannot be controlled with the precision required. Also, the plasma can cause undesired polymerization of the masking resist on a substrate layer, to make later removal difficult, if not impossible, without damage.